19
219 - 1 = 524287, Prime Number
1019 - 1 = 3 * 3 * 1111111111111111111
18 - 19 - 20
0 - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9
Mathematics Encyclopedia
Hellenica World - Scientific Library
